Resistance material and method of making same



y 1931. v E. J. HAVERSTICK 1,806,347

RESISTANCE MATERIAL AND METHOD OF MAKING SAME Filed July 5, 1926 Gfdmg Z mpb/acA per /00 Gram,- f7/r27,

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05/0 5 Grams dampb/ac-K WITNESSES! I INVENTOR I a fbr/JHa/QIZifiCK m Wufl BY 7 ATTORNEY Patented May 19, 1931 UNITED STATES PATENT OFFICE EARL J. HAVERSTICK, OF OAIKMONT, PENNSYLVANIA, ASSIGNOR TO WESTINGHOUSE ELECTRIC & MANUFACTURING COMPANY, A CORPORATION OF PENNSYLVANIA RESISTANCE MATERIAL AND METHOD OF MAKING SAME Application filed m a, 1926. Serial no. 120,457.

My invention relates to resistor material and particularly to a method of manufacturemetallic resistors of ceramic materials, in-

cluding kaolin or other binders; conducting materials, such as graphite or carborundum; and filler materials such as silica, in such proportions as to provide the proper resistance.

It has been found in practice that resistors designed to give a resistance of 70 ohms per centimeter cube and above were diflicult of control in the process of manufacture with regard to the resistance of the finished product. This difficulty rapidly became more troublesome in resistors having a resistance above 100 ohms per centimeter cube. This condition made the satisfactory manufacture of resistors with a resistance of over 150 ohms per centimeter cube extremely difficult, if not impossible.

It was, therefore, necessary that some meth- 0d of manufacture be found that would allow better control of the product. My invention provides a process whereby the disadvantages of prior methods are minimized.

M invention utilizes a peculiar action whic takes place upon mixing two materials. If we take a given quantity of flint and mix in a given quantity of lamp black, the resulting resistor material will have a certain specific resistance. If, however, we take only a part of the given quantity of flint, mix in the given part of lamp black and then mix in the remaining part of flint, we obtain another specific resistance, yet the quantities of flint and lamp black are the same as before. This unexpected action ill-mixing constitutes an important part of my invention.

The probable explanation of this action is as follows:

If two powders of different size are mixed together the one of larger diameter will be coated by the other. It is evident from this fact that if we mix two materials, one an inert, the other a conducting material and of smaller diameter than the first, the conducting material tends to coat the inert material. If for any given percentage composition, all the conducting material is mixed with a portion of the inert material, particles of a larger diameter than the original particles will be formed. Upon addition of the remainder of the inert material, it will tend to coat these larger particles and have the effect of increasing the resistivity of the final m1x.

My invention may be better understood by reference to the accompanying drawings, in which the curves of Fig. 1 and Fig. 2- show relations between the amounts of flint and lamp black and the resistance of the resulting mixture.

Fig. 1 represents the results obtained by methods of the prior art. The number of grams of lamp black per 100 grams of flint is plotted against resistivity in ohms per cubic centimeter.

Referring to Fig. 1 suppose a resistor is desired having a resistance of 70 ohms per centimeter cube as indicated at point 2. Passinghorizontally to the curve at point 3 and vertically downward to point 4, we find that in order to obtain the desired resistivity we must use a ratio of 4.5 grams of lamp black per 100 grams of flint. It will be noted that at this point 3, and, in fact, along the portion of the curve between points 3 and 6, for a certain mix we obtain a relatively definite resistivity. If, however, we take some point 7 along the steeper portion of the curve between points 3 and 9, say, corresponding to a resistivity of 150 ohms per centimeter cube, 2. small change in'the mix, corresponding to the space between 8 and 11, produces a relatively large change in resistivity, as indicated by point 10. It can readily be understood tained by using my method. In this case 5 grams of lamp black were used. Similar curves will be obtained with other proportions of lamp black and other inert materials, such as flint. The 100-gram portion of the inert material which in the example given is flint is now divided into two parts. The curve indicating the portion in grams of flint to be mixed at the first mixin with a given quantity of lamp-black to obtam, when mlxed with the remaining ortion of flint, a certain desired resistance. uppose, as before, a resistivity of 150 ohms per centimeter cube is desired. Passing horizontally to the curve at point 12 and vertically downward to point 13 we see that 40 grams of flint are used in the first or preliminary portion, 5 grams of lamp-black are added to I this portion. A small amount of water is added to facilitate mixing. It is to be noted that 5 grams of lampblack are used instead of 3.4 as in Fig. 1, point 8. This increase in the amount of the coating material is advantageous as it provides a better coating and thus more uniformity is secured. After the preliminary mix is com leted, the remaining portion, or 60 grams of flint, is added and the final mix completed. This material may be treated with water glass, formed, again heated and baked to form'resistors as described in the copending application of Joseph Slepian, Serial N o. 637 ,147 or my application Serial No. 637,376, both filed May 7, 1923, and both of which are assigned to the within named assignee. By reference to the curve at point 12, it will be readily understood that the resistance of the resistors formed according to my process maybe relatively accurately predicted.

My invention thus provides a method whereby, in manufacture, the resistance of the resulting resistor may be readily controlled. The range of dependable resistances is greatly increased. In making a resistor of a desired resistance it is only necessary to choose the portions of material best suited therefor, as shown by the curves.

I have described a preferred method of utilizing my invention and have mentioned preferred materials. However, other materials may be used, such as silica, flint, carborundum and other silicious materials, on the one hand, and lampblack, graphite or other finely divided carbonaceous materials, on the other, and will act in the same way. It is understood that I do not wish to limit myself to the specific methods and materials described and that variations thereof may be made without departing from the spirit invention as defined in the adding another portion of flint, mixing, adding a binder, forming and baklng.

2. A method of manufacturing a resistor consisting of first mixing a portion of flint with a portion of lampblack and water, second, mixing in another portion of flint, and third, adding a binder, forming and baking.

3. A method of manufacturing a resistor consisting of mixing successive portions of flint, lampblack and flint with each other.

4. A method of manufacturing a resistor by mixing successive portions of flint, lampblack and flint with each other, adding a binder, forming and baking.

5. The method ofcontrolling the resistivity of a resistor element consisting of a composite mixture of lampblack and finely divided flint which comprises adding lampblack to a part of said flint, preliminarily mixing said flint and lampblack, mixing said preliminary mixture with the remainder of said flint, adding an adhesive binder and molding.

6. The method of controlling the resistivities of composite resistor bodies which consists in mixing together powdered substantially non-conductive material and a conducting material in such manner that the 001111110. tive material becomes coated upon the surface of the loose particles of the substantially non-conductive material, adding to the result ing mix some more powdered substantially non-conductive material, mixing the same in with the first mix in such manner that some of the added non-conducting material becomes coated upon the surface of the previously coated loose particles, and causing said doubly coated particles to be bonded together into a solid.

7. The method of controlling the resistivities of composite resistor bodies which consists in mixing together, without any substantial amount of liquid, a powdered mate rial and a second, more finely powdered, material having a higher order of conductivity than said first-mentioned material, adding to the resulting mix some additional powdered material having a lower order of conductivity than said second-mentioned material, mixing the materials a second time, and causing the particles resulting from said second mix to be bonded together intoa solid.

8. The method of controlling the resistivities of composite resistor bodies which consists in mixing together, without any substantial amount of liquid, a powdered material and a second, more finely powdered, material havin a higher order of conductivity than said rst-mentioned material, adding tothe resulting mix'some additional pow- 

